Performing a cyclic redundancy checksum operation responsive to a user-level instruction

ABSTRACT

In one embodiment, the present invention includes a method for receiving incoming data in a processor and performing a checksum operation on the incoming data in the processor pursuant to a user-level instruction for the checksum operation. For example, a cyclic redundancy checksum may be computed in the processor itself responsive to the user-level instruction. Other embodiments are described and claimed.

This application is a continuation of U.S. patent application Ser. No.13/796,032, filed Mar. 12, 2013, which is a continuation of U.S. patentapplication Ser. No. 13/484,787, filed May 31, 2012, which is now U.S.Pat. No. 8,413,024 issued on Apr. 2, 2013, which is a continuation ofU.S. patent application Ser. No. 13/097,462, filed Apr. 29, 2011, whichis now U.S. Pat. No. 8,225,184 issued on Jul. 17, 2012, which is acontinuation of U.S. patent application Ser. No. 11/316,772, filed Dec.23, 2005, which is now U.S. Pat. No. 7,958,436 issued on Jun. 7, 2011,the content of which is hereby incorporated by reference.

BACKGROUND

Embodiments of the present invention relate to data processing, and moreparticularly to determining checksums such as cyclic redundancy checks(CRCs).

In data processing systems, it is desirable that data transmittedbetween a first location and a second location is received accurately,so that additional processing performed on that data at the secondlocation also can be accurate. Further, to enable detection of errors indata transmission, oftentimes a data packet will be transmitted with achecksum attached. For example, a CRC sum can be generated by atransmitting source and appended to data to be transmitted. Thischecksum, which may be calculated according to one of many differentalgorithms, can then be compared to a similar checksum generated at thereceiving end from the received data. If the two checksums areidentical, the transmitted data is correct. If however the generatedchecksum varies from the transmitted checksum, an error is indicated.Such checksums are used throughout networking technologies to detecttransmission errors.

In different applications, different manners of implementing CRCinformation exists. For example, CRC calculations can be performed ineither hardware or software. To implement a CRC calculation in hardware,typically a dedicated hardware engine is provided within a system toperform the CRC calculation. Accordingly, data to be subjected to such aCRC calculation is sent to the hardware engine for calculation of theCRC, which is then appended to the data, e.g., for transmission from thesystem. Various drawbacks exist to using such an offload engine,including the overhead of sending data to the engine. Furthermore, it isdifficult to perform a stateless hardware offload. That is, typicallyadditional state-based overhead data also needs to be transmitted,increasing complexity and slowing the progress of useful work.

Because many systems lack such an offload engine, CRC calculations areoften performed in software. To implement CRC calculations in software,typically lookup table schemes are used. However, such softwarecalculations of CRC values are notoriously slow, compute-intensiveoperations. Further, the memory footprint of the lookup table can belarge, impacting performance. Accordingly, these slow calculations candegrade network performance, and further consume processing resources.As an example, it can take between 5 and 15 cycles to perform a CRCcalculation per byte of data. As a result, software CRC performance istoo low for general use in high-speed networks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a method in accordance with one embodimentof the present invention.

FIG. 2 is a block diagram of a processor in accordance with oneembodiment of the present invention.

FIG. 3 is a block diagram of a portion of a processor to perform achecksum operation in accordance with an embodiment of the presentinvention.

FIG. 4 is a block diagram of another portion of a processor inaccordance with an embodiment of the present invention.

FIG. 5 is a block diagram of a system in accordance with an embodimentof the present invention.

DETAILED DESCRIPTION

In various embodiments, checksum operations may be effected using aninstruction set architecture (ISA) extension to compute checksum values.More specifically, a user-level instruction may be provided within anISA to enable a programmer to directly perform a desired checksumoperation such as a CRC operation in a general-purpose processor (e.g.,a central processor unit (CPU)) via the instruction. The CRC operationmay be a 32-bit CRC operation (i.e., a CRC32 operation generating a32-bit running reminder, discussed further below), and in differentembodiments may, for example, correspond to the CRC used in an Instituteof Electrical and Electronics Engineers (IEEE) 802.3 Ethernet protocol(published 2002) or other protocols.

In different implementations, various opcode instructions may beprovided to perform CRC computations on different groupings of data. Forexample, in some embodiments CRC computations may be supported on groupsof 8, 16, 32 and 64 bits using different opcodes, although the scope ofthe present invention is not so limited. In this way, CRC calculationsmay be rapidly performed in hardware without the need for lookup tablesor the like. Furthermore, the computations may be performed usinggeneric, architecturally visible processor registers via integeroperations performed according to the different opcodes. As a result,CRCs may be computed in a processor without the need for the overheadand complexity of offload hardware, such as network offload hardware.Accordingly, greater numbers of data transmissions (e.g., in terms ofinput/outputs (I/Os) per second) can occur. Note that while describedprimarily herein in connection with CRC operations, embodiments of thepresent invention may be used to perform other checksum operations.

Referring now to FIG. 1, shown is a flow diagram of a method inaccordance with one embodiment of the present invention. Method 100 maybe used to obtain a checksum using a user-level instruction implementedon processor hardware, e.g., an execution unit of a CPU. As shown inFIG. 1, method 100 may begin by performing a series of exclusive-OR(XOR) operations on data in source and destination registers (block110). Note that the XOR operations may correspond to a polynomialarithmetic operation and more particularly to a polynomial divisionoperation. The data in the source register may correspond, e.g., to datapresent in a processor pipeline that has been received by the processoror is to be transmitted therefrom. As an example, a group of data in abuffer corresponding to a desired group size (e.g., 16 bit, 32 bit orthe like) may be provided to the source register, which may be ageneral-purpose register of the processor. Alternately, the source datamay be obtained from a memory, in some embodiments. The destinationregister may correspond to a storage location for a running remainderobtained from the XOR operations. The destination register also may be ageneral-purpose register of the processor.

In various embodiments, the XOR operations may be performed in dedicatedhardware within a processor pipeline. For example, an execution unit ofa processor, e.g., an integer execution unit may be extended withcircuitry to implement a series of XOR operations. For example, thiscircuitry may correspond to a XOR tree to handle polynomial division bya desired polynomial. In various embodiments, a polynomial for use inthe XOR operations may be hard-wired into the logic gates of the XORtree. Furthermore, the XOR tree may be configured to implement desiredpre-processing and post-processing via the XOR operations, e.g., bitreflections and the like. Furthermore, the XOR tree logic may includemultiple partitions, each configured to handle operations on differentdata sizes.

Still referring to FIG. 1, next a result, which may correspond to arunning remainder obtained from the XOR operations, may be stored in thedestination register (block 120). Note that the destination registermay, upon initialization of a system, be set to a predetermined value,e.g., all ones, all zeros or another such value. Then during executionof checksum operations, this running remainder is continually updatedwith the result of the current checksum operation. More specifically,the remainder of the polynomial division implemented by the currentchecksum operation may be stored in the destination register.

Next, it may be determined whether additional source data is present(diamond 130). For example, in some embodiments a buffer may includedata that has been received by a system and is to have a checksumverified. The data may be fed in chunks into the source register toeffect the checksum operation. Accordingly, it may be determined indiamond 130 if additional source data is present in this buffer. If so,the next data chunk may be provided to the source register, and controlpasses back to block 110, discussed above.

If instead at diamond 130 it is determined that no additional sourcedata is present, control passes to block 140. There, the result of thechecksum operation may be provided as the current value (e.g., runningremainder) that is stored in the destination register (block 140). Asdiscussed above, this checksum value may be used in many differentmanners. For example, in the case of received data, the computedchecksum may be compared to a received checksum to confirm that the datawas accurately received. In a transmission situation, the checksum maybe appended to data to be transmitted so that the data may be verifiedon a receiving end. Of course other uses of checksums, such as for hashfunctions or generation of numbers pursuant to a pseudo random numberingscheme may also occur.

A processor to implement checksum operations in accordance with anembodiment of the present invention may take many different formsdepending on a desired architecture. Referring now to FIG. 2, shown is ablock diagram of a processor in accordance with one embodiment of thepresent invention. As shown in FIG. 2, processor 200 includes a datapath 205. Data path 205 may be controlled by front end control stagesthat may include a register alias table (RAT) 270, which may receivedecoded instructions from a front end of the processor (not shown inFIG. 2). RAT 270 may be used to receive microoperations (pops) from thefront end and rename the pops for the resources of the data path. Indata path 205, the renamed pops may then be provided to a reorder buffer(ROB) 250. ROB 250 may act as a register file to store pops andcorresponding source operands until the pop is ready for passing to areservation station (RS) 230. Similarly, ROB 250 may also storecorresponding results of pops that have already executed. These resultsmay be held in ROB 250 until the pops are retired (at which point theROB entry is freed).

Reservation station 230 may be used to store pops until theircorresponding source operands are present and/or until the pop is readyfor execution in one of a plurality of execution units of data path 205.Reservation station 230 may include a plurality of dispatch ports tocouple instructions and data to selected ones of execution units of datapath 205. In some embodiments, multiple dispatch ports may be used ineach cycle.

As shown in FIG. 2, the execution units in data path 205 include anaddress generation unit (AGU) 220, an integer (INT) execution unit 222,a store data (STD) unit 224, a floating point (FP) execution unit 226,and a single instruction multiple data (SIMD) execution unit 228. Asshown in FIG. 2, integer execution unit 222 further includes logic 221.Logic 221 may include one or more hardware engines to perform checksumoperations in accordance with an embodiment of the present invention.More specifically, logic 221 may include a plurality of exclusive-OR(XOR) logic trees to implement polynomial arithmetic and related datamanipulations. In various embodiments, logic 221 may include differenthardware engines to implement CRC operations on differently sized datachunks. As an example, a plurality of user-level instructions of an ISAeach may define a CRC operation for a particular data size. Logic 221,in some embodiments, may include a corresponding number of separatehardware engines, also referred to herein as XOR trees, to effect thesedifferent CRC operations.

While not shown in FIG. 2, additional or different execution units maybe present in different embodiments. After execution of a pop in one ofthe execution units, result data may be passed back to RS 230 and ROB250 for storage, e.g., until retirement. Thus in one embodiment, bothsource and data registers for performing a CRC operation may be locatedin RS 230 or ROB 250. While not shown in FIG. 2, it is to be understoodthat additional buffers such as a memory order buffer (MOB) and otherresources may be present within processor 200.

It is further to be understood that the representation shown in FIG. 2is intended for ease of discussion and in various embodiments many morestages or differently named stages may exist in a given processor. Forexample, a write back stage may be coupled to the execution units toreceive result data for later delivery to a memory hierarchy.Alternately, one or more other buffers such as store buffers, loadbuffers and the like may be coupled to RS 230. As one example, one ormore retirement buffers may be coupled to RS 230 for storage of pops andassociated result data until retirement of the associated instruction.

Of course, other implementations are possible. Referring now to FIG. 3,shown is a block diagram of a portion of a processor to perform achecksum operation in accordance with an embodiment of the presentinvention. As shown in FIG. 3, a portion of a processor 300 is shown.More specifically, processor 300 includes an XOR tree 310, a firstregister 320 and a second register 330, all of which may be part of aprocessor pipeline. XOR tree 310 may be configured differently invarious embodiments. For example, XOR tree 310 may be implemented usinga plurality of 3-input XOR gates in a first level, outputs of which arecoupled to similar XOR gates of a second level, and so forth. In such anembodiment, each level of the XOR tree may be a third as large as theprevious level. Of course, other configurations are possible.

As further shown in FIG. 3, processor 300 includes a buffer 340, whichalso may be within the processor pipeline (e.g., as a buffer, queue orthe like). Alternately, buffer 340 may be a cache memory associated withprocessor 300. In the embodiment of FIG. 3, first register 320 maycorrespond to a source register, while second register 330 maycorrespond to a destination register. In various embodiments, theseregisters may be general-purpose registers within processor 300. Ofcourse, processor 300 may include many other registers, logic,functional units and the like, and the portion shown in FIG. 3 is forease of illustration.

As shown in FIG. 3, to perform a checksum in accordance with anembodiment of the present invention, at least a first portion of firstregister 320 is provided to XOR tree 310, along with a portion of secondregister 330. In the embodiment shown in FIG. 3, which illustrates an8-bit CRC accumulation, a single byte of data (B₀) is provided to XORtree 310 from first register 320, while a 4-byte portion of secondregister 330 is provided to XOR tree 310. This 4-byte portion maycorrespond to the running remainder of a CRC32 operation. Using thisdata, XOR tree 310 may perform data manipulations via XOR operations togenerate a result that includes a remainder portion. This remainderportion may be the running remainder that is stored back in secondregister 330, as shown in FIG. 3. In this way, CRC operations can beefficiently performed in minimal cycle time and using minimal processorresources. In the embodiment of FIG. 3, for 8-bit accumulate operations,additional portions of first register 320 may be provided incrementallyto XOR tree 310 along with the current contents of second register 330(i.e., the 32-bit running remainder). Accordingly, to obtain a CRCchecksum on 64 bits of data in first register 320, eight iterations ofXOR operations in XOR tree 310 may be performed, each using a singlebyte of data from first register 320, along with the current runningremainder in second register 330. If additional data is present inbuffer 340 to be validated via a checksum, the additional data may beloaded into first register 320 so that it may then be processed in XORtree 310.

Note that different hardware may be present to handle CRC calculationsof different bit widths. Accordingly, with reference back to FIG. 2,logic 221 may include different XOR tree structures to handle such CRCcalculations. Referring now to FIG. 4, shown is a block diagram ofanother portion of a processor in accordance with an embodiment of thepresent invention. As shown in FIG. 4, processor 300 includes adifferent XOR tree 410 (e.g., in addition to XOR tree 310 of FIG. 3)that is coupled to receive data from first register 320 and secondregister 330. As further shown in FIG. 4, buffer 340 is present and maybe used to provide data for CRC computations. Note that in theembodiment of FIG. 4, XOR tree 410 is configured to handle a 64-bit CRCaccumulation. Accordingly, the entire contents of first register 320(i.e., bytes B₀-B₇) may be coupled at one time to XOR tree 410 formanipulation in XOR operations with data in second register 330. Theresult data, the desired portion of which corresponds to a runningremainder, is stored back in second register 330. While described withthese particular implementations in FIGS. 3 and 4, it is to beunderstood that the scope of the present invention is not so limited,and in other embodiments different hardware configurations forperforming CRC operations may be present.

Referring now to Table 1 below, shown is a listing of exampleinstructions of an instruction set architecture (ISA) to support CRCoperations in accordance with various embodiments of the presentinvention. As shown in Table 1, each instruction, which may bereferenced by an opcode, is used to perform a CRC32 operation using asource register and a destination register. As shown, differs flavorsare possible, with each instruction to perform the CRC operation on agiven size of destination operand and source operand. Thus withreference to the first line of Table 1, this instruction is used toperform a CRC32 operation on an 8-bit source operand and a 32-bitdestination operand. Similarly, the second line of Table 1 is used toperform a CRC32 operation on a 16-bit source operand and a 32-bitdestination operand. In similar fashion, the third line of Table 1 showsan instruction to perform a CRC32 operation on a 32-bit source operandand a 32-bit destination operand.

Because these first three instructions are performed with maximum datachunks of 32 bits, note that the instructions are valid in both a 64-bitmode of operation as well as a legacy (i.e., 32-bit) mode of operation.In contrast, the fourth and fifth lines of Table 1 denote CRC operationsto be performed on 8-bit and 64-bit source operands, respectively with a64-bit destination operand. Thus these final two instructions may beperformed only in a 64-bit mode of operation.

TABLE 1 Opcode Instruction Description Code 2 CRC32 r32, r/m8 AccumulateCRC32 on r/m8 Code 1 CRC32 r32, r/m16 Accumulate CRC32 on r/m16 Code 1CRC32 r32, r/m32 Accumulate CRC32 on r/m32 Code 2 CRC32 r64, r/m8Accumulate CRC32 on r/m8 Code 1 CRC32 r64, r/m64 Accumulate CRC32 onr/m64In various embodiments, these user-level instructions may be used by aprogrammer, e.g., as intrinsics to implement a CRC operation inaccordance with the flow diagram of FIG. 1, for example.

In general, a user-level CRC instruction may be implemented in thefollowing manner. Starting with an initial value in a first operand(i.e., a destination operand), a CRC32 value for a second operand (i.e.,a source operand) may be accumulated and the result stored back in thedestination operand. In different implementations, the source operandcan be a register or a memory location. The destination operand may be a32 or 64-bit register. If the destination is a 64-bit register, then the32-bit result may be stored in the least significant double word and00000000H stored in the most significant double word of the register.

Note that the initial value supplied in the destination operand may be adouble word integer stored in a 32-bit register, or the leastsignificant double word of a 64-bit register. To incrementallyaccumulate a CRC32 value, software retains the result of the previousCRC operation in the destination operand, and then executes the CRCoperation again with new input data in the source operand. Accordingly,each instruction takes a running CRC value in the first operand andupdates the CRC value based on the second operand. In this manner, a CRCcan be generated over any desired amount of data by performing theoperation in a loop, until all desired data is subjected to the CRCoperation.

In some implementations, data contained in the source operand isprocessed in reflected bit order. This means that the most significantbit of the source operand is treated as the least significant bit of thequotient, and so on, for all the bits of the source operand. Likewise,the result of the CRC operation can be stored in the destinationregister in reflected bit order. This means that the most significantbit of the resulting CRC (i.e., bit 31) is stored in the leastsignificant bit of the destination register (bit 0), and so on, for allthe bits of the CRC.

While different manners of implementing these user-level instructionscan be effected, Tables 2-6 below show example pseudocoderepresentations of a hardware implementation for each of the user-levelinstructions of Table 1.

TABLE 2 CRC32 instruction for 64-bit source operand and 64-bitdestination operand: TEMP1[63-0] ← BIT_REFLECT64 (SRC[63-0]) TEMP2[31-0]← BIT_REFLECT32 (DEST[31-0]) TEMP3[95-0] ← TEMP1[63-0] << 32 TEMP4[95-0]← TEMP2[31-0] << 64 TEMP5[95-0] ← TEMP3[95-0] XOR TEMP4[95-0]TEMP6[31-0] ← TEMP5[95-0] MOD2 11EDC6F41H DEST[31-0] ← BIT_REFLECT(TEMP6[31-0]) DEST[63-32] ← 00000000H

TABLE 3 CRC32 instruction for 32-bit source operand and 32-bitdestination operand: TEMP1[31-0] ← BIT_REFLECT32 (SRC[31-0]) TEMP2[31-0]← BIT_REFLECT32 (DEST[31-0]) TEMP3[63-0] ← TEMP1[31-0] << 32 TEMP4[63-0]← TEMP2[31-0] << 32 TEMP5[63-0] ← TEMP3[63-0] XOR TEMP4[63-0]TEMP6[31-0] ← TEMP5[63-0] MOD2 11EDC6F41H DEST[31-0] ← BIT_REFLECT(TEMP6[31-0])

TABLE 4 CRC32 instruction for 16-bit source operand and 32-bitdestination operand:: TEMP1[15-0] ← BIT_REFLECT16 (SRC[15-0])TEMP2[31-0] ← BIT_REFLECT32 (DEST[31-0]) TEMP3[47-0] ← TEMP1[15-0] << 32TEMP4[47-0] ← TEMP2[31-0] << 16 TEMP5[47-0] ← TEMP3[47-0] XORTEMP4[47-0] TEMP6[31-0] ← TEMP5[47-0] MOD2 11EDC6F41H DEST[31-0] ←BIT_REFLECT (TEMP6[31-0])

TABLE 5 CRC32 instruction for 8-bit source operand and 64-bitdestination operand: TEMP1[7-0] ← BIT_REFLECT8(SRC[7-0]) TEMP2[31-0] ←BIT_REFLECT32 (DEST[31-0]) TEMP3[39-0] ← TEMP1[7-0] << 32 TEMP4[39-0] ←TEMP2[31-0] << 8 TEMP5[39-0] ← TEMP3[39-0] XOR TEMP4[39-0] TEMP6[31-0] ←TEMP5[39-0] MOD2 11DC6F41H DEST[31-0] ← BIT_REFLECT (TEMP6[31-0])DEST[63-32] ← 00000000H

TABLE 6 CRC32 instruction for 8-bit source operand and 32-bitdestination operand: TEMP1[7-0] ← BIT_REFLECT8(SRC[7-0]) TEMP2[31-0] ←BIT_REFLECT32 (DEST[31-0]) TEMP3[39-0] ← TEMP1[7-0] << 32 TEMP4[39-0] ←TEMP2[31-0] << 8 TEMP5[39-0] ← TEMP3[39-0] XOR TEMP4[39-0] TEMP6[31-0] ←TEMP5[39-0] MOD2 11EDC6F41H DEST[31-0] ← BIT_REFLECT (TEMP6[31-0])

Note that the general structure of these pseudocode snippets are thesame. First, data in a source register is bit reflected (i.e., its bitsare placed into a temporary register in reverse bit order). Thedestination register is similarly bit reflected. Next, shift operations,more particularly shift left operations, may be effected on both of thebit-reflected source and data operands. The resulting values may then besubjected to an XOR operation. This operation may correspond to apolynomial division by a selected polynomial value. While this value maytake many different forms in different embodiments, in particularimplementations for performing CRC32 operations, the polynomial maycorrespond to 11EDC6F41H, although the scope of the present invention isnot so limited. The remainder of this polynomial division (i.e., theremainder from the polynomial division modulus 2) is stored back intothe low order bits of the destination operand in a bit-reflected order(e.g., bits 0-31 of either a 32-bit or 64-bit register). In the instanceof a 64-bit register, the most significant bits (MSBs) may be loadedwith zeros. While set forth with this particular implementation withrespect to Tables 2-6, it is to be understood that other manners ofproviding a user-level CRC instruction may be performed.

By performing CRC operations in a processor pipeline itself according toa user-level instruction, there is no need to send data to an offloadengine. Similarly, the operation can be performed without providingstate, reducing overhead. In this way, as implemented in a three-cyclepath a CRC operation may be performed at less than approximately 0.4cycles per byte. Accordingly, performance may be improved usinguser-level instructions along with dedicated hardware in a processorpipeline. Furthermore, three-cycle latency may be realized with minimumreal estate consumption and power consumption. Embodiments of thepresent invention may be used to enable processing of various storageprotocols, for example, an Internet Small Computer System Interface(iSCSI) protocol at rates greater than 10 gigabits per second.Embodiments of the present invention further allow the use of datapresent in a processor or closely coupled thereto, reducing the need foron-cache data. In this way, data in a processor buffer may be fed to anXOR tree to enable rapid, on-the-fly CRC calculations.

Embodiments may be implemented in many different system types. Referringnow to FIG. 5, shown is a block diagram of a multiprocessor system inaccordance with an embodiment of the present invention. As shown in FIG.5, the multiprocessor system is a point-to-point interconnect system,and includes a first processor 470 and a second processor 480 coupledvia a point-to-point interconnect 450. As shown in FIG. 5, each ofprocessors 470 and 480 may be multicore processors, including first andsecond processor cores (i.e., processor cores 474 a and 474 b andprocessor cores 484 a and 484 b). While not shown for ease ofillustration, first processor 470 and second processor 480 (and morespecifically the cores therein) may include XOR tree logic within theirexecution units to execute user-level CRC instructions in accordancewith an embodiment of the present invention. First processor 470 furtherincludes a memory controller hub (MCH) 472 and point-to-point (P-P)interfaces 476 and 478. Similarly, second processor 480 includes a MCH482 and P-P interfaces 486 and 488. As shown in FIG. 5, MCH's 472 and482 couple the processors to respective memories, namely a memory 432and a memory 434, which may be portions of main memory locally attachedto the respective processors.

First processor 470 and second processor 480 may be coupled to a chipset490 via P-P interconnects 452 and 454, respectively. As shown in FIG. 5,chipset 490 includes P-P interfaces 494 and 498. Furthermore, chipset490 includes an interface 492 to couple chipset 490 with a highperformance graphics engine 438. In one embodiment, an Advanced GraphicsPort (AGP) bus 439 may be used to couple graphics engine 438 to chipset490. AGP bus 439 may conform to the Accelerated Graphics Port InterfaceSpecification, Revision 2.0, published May 4, 1998, by IntelCorporation, Santa Clara, Calif. Alternately, a point-to-pointinterconnect 439 may couple these components.

In turn, chipset 490 may be coupled to a first bus 416 via an interface496. In one embodiment, first bus 416 may be a Peripheral ComponentInterconnect (PCI) bus, as defined by the PCI Local Bus Specification,Production Version, Revision 2.1, dated June 1995 or a bus such as thePCI Express bus or another third generation input/output (I/O)interconnect bus, although the scope of the present invention is not solimited.

As shown in FIG. 5, various I/O devices 414 may be coupled to first bus416, along with a bus bridge 418 which couples first bus 416 to a secondbus 420. In one embodiment, second bus 420 may be a low pin count (LPC)bus. Various devices may be coupled to second bus 420 including, forexample, a keyboard/mouse 422, communication devices 426 and a datastorage unit 428 which may include code 430, in one embodiment. Further,an audio I/O 424 may be coupled to second bus 420. Note that otherarchitectures are possible. For example, instead of the point-to-pointarchitecture of FIG. 5, a system may implement a multi-drop bus oranother such architecture.

Embodiments may be implemented in code and may be stored on a storagemedium having stored thereon instructions which can be used to program asystem to perform the instructions. The storage medium may include, butis not limited to, any type of disk including floppy disks, opticaldisks, compact disk read-only memories (CD-ROMs), compact diskrewritables (CD-RWs), and magneto-optical disks, semiconductor devicessuch as read-only memories (ROMs), random access memories (RAMs) such asdynamic random access memories (DRAMs), static random access memories(SRAMs), erasable programmable read-only memories (EPROMs), flashmemories, electrically erasable programmable read-only memories(EEPROMs), magnetic or optical cards, or any other type of mediasuitable for storing electronic instructions.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A system comprising: a processor comprising: aset of registers, including: a first 32-bit register to store a firstoperand; a second 32-bit register to store a second operand; a first64-bit register to store a third operand; and a second 64-bit registerto store a fourth operand; a plurality of execution units to performexclusive-OR (XOR) operations on data of a configurable size responsiveto instructions of an instruction set architecture (ISA) for theprocessor, the plurality of execution units including: a first executionunit coupled to the first and the second 32-bit registers to perform afirst XOR operation on at least one bit of the first and the secondoperands and to store a result of the first XOR operation in a firstdestination register responsive to a first instruction of the ISA; and asecond execution unit coupled to the first and the second 64-bitregisters to perform a second XOR operation on at least one bit of thethird and the fourth operands and to store a result of the second XORoperation in a second destination register responsive to a secondinstruction of the ISA; and a memory controller; a memory coupled withthe memory controller; a data storage device coupled with the processor;an audio I/0 interface coupled with the processor; and a communicationdevice coupled with the processor;
 2. The system of claim 1, wherein thefirst and the second execution units are to perform the first and thesecond XOR operations in the same number of cycles.
 3. The system ofclaim 1, wherein the first destination register comprises the first32-bit register.
 4. The system of claim 1, wherein the first destinationregister comprises the second 32-bit register.
 5. The system of claim 1,wherein the second destination register comprises the first 64-bitregister.
 6. The system of claim 1, wherein the second destinationregister comprises the second 64-bit register.